This invention relates to a sealing ring structure useful for hydraulically operated pistons or piston rods. The invention is applicable to sealing ring structures comprising a tough elastic sealing ring which may be of a thermoplastic substance, e.g. based on polytetraflure ethylene and a separate highly elastic stressing ring for pressing radially on the sealing ring and preferably consisting of rubber, or to integral structures having a sealing section produced from an elastic material or a combination of materials to provide a tough elastic sealing section and integral highly elastic stressing section, the structure in either case being used as a contact seal between a stationary and a moving machine element, the sealing ring or the sealing section having an intrinsically elastic radial initial stress.
In the case of piston and piston rod seals of the above nature, it is necessary that a slight leakage be retained during reciprocating displacement. For low-friction sealing of reciprocating pistons and piston rods, use is made inter alia of two-piece seals, consisting of a synthetic material ring having a substantially rectangular cross-section for sealing the gap established by relative displacement between the machine elements displaceable with respect to each other, and of a rubber-elastic ring for secondary sealing and prestressing of the synthetic material ring. For the purpose of clear distinction, reference will be made hereinafter to the synthetic material ring establishing the moving seal as the sealing ring, and to the rubber-elastic ring thrusting the synthetic material ring into contact as a result of pressure-induced transverse thrust and over-dimensionally induced initial stress, as the stressing ring.
The sealing ring and stressing ring are commonly situated together within a groove or recess of the housing (for a rod seal) or of the piston (for a piston seal). The stressing ring commonly comprises a rubber-elastic ring of circular (O-ring) or rectangular cross-section, and its axis of radial symmetry is situated on an extension of the axis of radial symmetry of the rectangular sealing ring. As a result of radial prestressing of the stressing ring between the bottom of the groove and the external surface of the sealing ring situated opposite to the moved sealing gap which is the outer surface (in the case of the rod seal) or the inner surface (in the case of the piston seal), the secondary leakage path between the sealing ring and the bottom of the groove and the sealing ring is closed.
Practical experience has shown that the coefficients of friction of such combined seals are substantially smaller, above all upon starting up and at low stroke speeds, than in the case of conventional seals comprising rubber-elastic contact surfaces on the moving machine element.
Practical experience also indicates however that the combination consisting of a sealing ring having a substantially rectangular cross-section and of a stressing ring, has approximately the same leakage behaviour as conventional rubber-elastic contact seals. In this case too, a "drag" flow resulting in the forming of a lubricating film with intermittent separation of the sealing contact surfaces can occur during the reciprocating displacement as a result of the viscosity of the fluid which is to be sealed. As a consequence of the asymmetrical pressure distribution within the area of contact, a lubricating film of different thickness is commonly formed during the outward and return strokes, which can result in accumulation of fluid to be sealed off, at the low-pressure extremity of the sealing gap and thus in leakage.
It is an object of the present invention to reduce the leakage during reciprocating displacement of the piston rod or of the piston by appropriately forming and arranging the sealing ring, without increasing the frictional values.